JUBA —
South Sudan accused rebels on Sunday of mobilizing a feared ethnic militia for new attacks despite a government offer of a truce to end two weeks of conflict in the young state.

A 25,000-strong force of the “White Army” - made up largely of ethnic Nuer youths who dust their bodies with ash - was marching on the town of Bor, recaptured by loyalist forces last Tuesday, an army spokesman said.

“We are prepared to engage them,” Sudan People's Liberation Army spokesman Philip Aguer said by phone from South Sudan's capital, Juba, 190 km (120 miles) south of Bor by road.

Fighting has left at least 1,000 dead and split the east African country barely two years after it won independence from Sudan. It has also raised fears of an all-out civil war between the main Dinka and Nuer ethnic groups which could destabilize the fragile region.

The White Army rebels, loyal to former Vice President Riek Machar, were likely to clash with President Salva Kiir's troops near Bor within the next day, the army spokesman said.

Machar made no immediate comment on the rebel force or on the government's offer of a ceasefire on Friday.

Witnesses spoke of panicked civilians fleeing Bor to escape another round of bloodletting.

The scene of a massacre of Kiir's Dinka ethnic group in 1991 by Nuer fighters loyal to Machar, Bor was retaken by government troops after several days of heavy fighting.

The White Army are recognized by the ash, prepared from burnt cow dung, with which they cover themselves to ward off insects. They are armed with machetes and sticks as well as guns.

“They are a wildcard whose intervention in the theater of conflict outside Bor could ratchet up the conflict even further and also put at even greater risk the lives of innocent civilians,” Joe Contreras, a spokesman for the U.N. mission in Sudan, told the BBC.

The army said rebels were also mobilizing youths and armed civilians for another attack on Malakal, the capital of the oil-producing Upper Nile state. Rebels were pushed out of the town on Friday.

Among the civilians trying to escape Bor, capital of Jonglei State, was Juuk Mading.

“We are very scared,” Mading, a father of four, said from a crowded river jetty as he waited in the fierce heat for a boat to cross the White Nile river to a neighboring state.

A Reuters cameraman who visited Bor on Dec. 25, a day after the rebels were pushed out, said burnt corpses lay in the streets. Traumatized civilians spoke of mass killings by marauding youths.

As well as offering a truce, President Kiir's government said it would release eight of 11 senior politicians, widely seen to be Machar allies, arrested over an alleged coup plot against Kiir.

Defense Minister Kuol Manyang Juuk told Reuters politicians in Juba were speaking to the White Army members to tell them that this is not an ethnic-based conflict and to convince them to abandon their march on Bor.